Known conventional processes for the preparation of thiazolecarboxylic acid chlorides include the process using thionyl chloride (Journal of Chemical Society, 1945, 601), (Journal of Heterocyclic Chemistry, 22, 1621 (1985)) and the process employing oxalyl chloride (Journal of Chemical Society Perkin 1, 159, (1982).
In the process making use of thionyl chloride, one or more side reactions occur when the starting thiazolecarboxylic acid is a 2-alkylthiazolecarboxylic acid. It is therefore necessary to maintain the reaction temperature at an extremely low level. A further limitation is imposed on the solvent. The yield is however extremely low even under such reaction conditions.
When a starting material other than 2-alkyl-thiazolecarboxylic acids is employed on the other hand, thionyl chloride undergoes an extremely vigorous reaction with the thiazolecarboxylic acid as the starting material so that the reaction can hardly be controlled. Further, the reaction is accompanied by rapid evolution of heat and immense production of corrosive sulfurous acid gas. As an industrial preparation process, this process therefore involves problems such that extra apparatus and labor are required for the processing of the sulfurous acid gas produced in a large volume.
The process in which oxalyl chloride is used is uneconomical for industrial applications because oxalyl chloride is costly. In addition, this process requires a long reaction time unless a catalyst is employed.